The body has a greater sensitivity to 15 Hz than it does to higher frequencies above 1000 Hz.
Also, the harmonics produce by the higher frequencies are stronger.
As I previously wrote; "bacteria such as anthrax, chlamidia, shigella, and e. choli have spectral responses between 392 and 398 kHz. This would be near the 13th harmonic of 30 kHz, and near the 157th and 159th harmonic of 2.5 kHz. The 13th harmonic has an amplitude of 1/13th of the main frequency and the 157th harmonic has an amplitude of 1/157th of the main frequency. The tradeoff of amplitude vs. closeness to the needed frequency has to be weighed in considering desired results."